1. Field of the Invention
The present invention relates to a depleted UF6 processing plant for processing depleted UF6 by converting UF6 into U3O8, and a method for processing depleted UF6.
2. Description of the Related Art
The proportion of depleted UF6 accumulated in an uranium enrichment plant amount to nearly 90% of the UF6 starting material, and it is mostly stored by filling in a UF6 cylinder that is a cylindrical sealed storage vessel. However, since this substance is almost permanently stored, there arises a management problem of maintaining the vessel with a large quantity of depleted UF6 from corrosion over an extended period of time, as well as waste of resources and economical deficiencies caused by a vast amount of fluorine resources being stored in the form of UF6.
A large amount of depleted UF6 containing a low concentration of U235 is accumulated in the enrichment process of U235 in the uranium enrichment plant when U235 is enriched using UF6 produced from natural uranium or recovered UF6 as a starting material. To solve the problems described, the inventors of the present invention proposed a method for processing depleted UF6 by converting depleted UF6 containing a low concentration of U235 into U3O8 by a dry vapor-phase reaction method (Japanese Unexamined Patent Publication No. 11-79749). The method for processing depleted UF6 comprises: extracting anhydrous hydrogen fluoride as a by-product using concentrated sulfuric acid and separating hydrogen fluoride from dilute sulfuric acid by distillation; further distilling and concentrating dilute sulfuric acid to separate dilute hydrofluoric acid and concentrated sulfuric acid; recycling this concentrated sulfuric acid to the extraction and concentration step while further distilling dilute hydrofluoric acid to separate it into azeotropic hydrofluoric acid and water that contains a small amount of hydrofluoric acid; and mixing azeotropic hydrofluoric acid with dilute hydrofluoric acid in the distillation and concentration step to improve recovery of hydrogen fluoride for recycling it in the nuclear facilities.
However, two distillation columns and one concentration column are required to regenerate hydrogen fluoride by the processing process of depleted UF6 described above in the nuclear facilities. In recycling hydrogen fluoride in the existing nuclear facilities, equipments related to the recycling should be additionally installed, resulting that supply of anhydrous hydrogen fluoride does not match demands for it. Accordingly, hydrogen fluoride generated as a by-product when depleted UF6 is converted into U3O8 is also desired to be recovered and stored since it can be readily recycled.
The method for recovering and storing fluorine known in the art includes forming calcium fluoride by a fixing reaction of fluorine to calcium, followed by storage of calcium fluoride. The inventors of the present invention proposed a method for recovering granular calcium fluoride by allowing a solution mainly containing hydrogen fluoride to contact granular calcium carbonate, and an equipment to be used for the method (Japanese Unexamined Patent Publication No. 10-330113). This equipment comprises a storage tank for storing a solution containing 10 to 60% of hydrogen fluoride, a first cooler for cooling the solution stored in the storage tank to 0 to 5xc2x0 C., a reaction tank for forming a solution containing granular calcium fluoride by adding granular calcium carbonate to the solution at a temperature of 0 to 5xc2x0 C. fed from the storage tank, and a solid/liquid separator for separating granular calcium fluoride from the solution containing it. This method is so devised that fluorine is recovered with a high yield by forming calcium fluoride by cooling the reaction solution to 0 to 5xc2x0 C. in the first cooler.
However, the hydrogen fluoride gas generated as a by-product when UF6 is converted into U3O8 is once turned into an aqueous hydrogen fluoride solution containing 10 to 60% of hydrogen fluoride, in order to recover hydrogen fluoride as a by-product using the equipment disclosed in Japanese Unexamined Patent Publication No. 10-330113. The foregoing conversion process requires additional facilities to be installed. It is also a problem in the conventional process described above that the recovery work becomes much complicated if hydrogen fluoride generated as a by-product in converting UF6 into U3O8 could not be recovered. Also, there is a drawback that calcium fluoride formed by the recovery of hydrogen fluoride tend to be a fine powder.
Accordingly, one object of the present invention is to provide a depleted UF6 processing plant which simplifies its facilities and processes while preventing recovered calcium fluoride from being a fine powder, and a method for processing depleted UF6.
One aspect of the present invention provides a depleted UF6 processing plant including a first fluidized bed reactor constructed so that UO2F2 and hydrogen fluoride is formed by allowing depleted UF6 to react with steam, and a second fluidized bed reactor constructed so that U3O8, hydrogen fluoride and oxygen is formed by allowing UO2F2 to further react with steam.
The processing plant further includes a gas cooler for cooling hydrogen fluoride generated in the first and second fluidized bed reactors to 150 to 300xc2x0 C., and a fluorine fixing reactor, which is filled with granular calcium carbonate 13a so that hydrogen fluoride cooled to 150 to 300xc2x0 C. by the gas cooler passes through, and forms granular calcium fluoride when hydrogen fluoride passing through the fluorine fixing reactor makes contact with granular calcium carbonate.
In the plant as described above, gaseous hydrogen fluoride generated in the first and second fluidized bed reactors is directly introduced into the fluorine fixing reactor. UF6 is converted into U3O8 simultaneously while forming granular calcium fluoride by allowing hydrogen fluoride to contact calcium carbonate in the fluorine fixing reactor, thereby recovering hydrogen fluoride as a by-product.
Calcium carbonate filled in the fluorine fixing reactor preferably has a grain size of 350 to 800 xcexcm. When the grain size is less than 350 xcexcm, hydrogen fluoride flow is inhibited while, when the grain size exceeds 800 xcexcm, the total surface area of calcium carbonate diminishes, thereby reducing the amount of calcium fluoride formed.
It is preferable that a plurality of cylinders in which calcium carbonate is housed are arranged, for example, on a circle so as to be exchangeable one another in the fluorine fixing reactor. After converting calcium carbonate into calcium fluoride by allowing hydrogen fluoride to pass through one or two of the plural cylinders, the cylinders in which calcium carbonate has been converted into calcium fluoride are replaced with another fresh cylinder to enable additional hydrogen fluoride to pass through the cylinders. A continuous processing of depleted UF6 is thus made possible when a fluorine fixing reactor capable of readily exchanging calcium carbonate is used.
The method for processing depleted UF6 preferably includes: a dry vapor-phase reaction step for forming UO2F2 by allowing depleted UF6 to react with steam at 230 to 280xc2x0 C., followed by forming U3O8, hydrogen fluoride and oxygen by allowing UO2F2 to further react with steam at 600 to 700xc2x0 C.; and a fluorine fixing step for forming granular calcium fluorides by allowing hydrogen fluoride generated in the dry vapor-phase reaction step to contact granular calcium carbonate at 150 to 300xc2x0 C.
In the method described above, UO2F2 grains with a mean grain size of 100 to 250 xcexcm and a bulk density of 3.5 g/cm2 or more, and hydrogen fluoride are formed by allowing depleted UF6 to react with steam by adjusting the reaction temperature at 230 to 280xc2x0 C.; and U3O8, hydrogen fluoride and oxygen are formed by further allowing the UO2F2 grains having the properties as described above to react with steam by adjusting the reaction temperature at 600xc2x0 C. or more. U3O8 thus formed has an approximately uniform mean grain size and an increased bulk density by about 10%, besides having good fluidity and being easy in handling to improve storage efficiency.
The granular shape of the calcium fluoride grains can be prevented from being collapsed by allowing gaseous hydrogen fluoride to directly contact the granular calcium carbonate and maintaining the temperature above the boiling point, or at 150 to 300xc2x0 C., of hydrogen fluoride formed in the dry vapor-phase reaction step, thus making handling of calcium fluoride formed by the fluoride fixing reaction easy.
In accordance with another aspect of the present invention, the dilute hydrofluoric acid formed in the fluorine fixing step described above is used for the steam to be used in the dry vapor-phase reaction process.
In the method as described above, efflux of the secondary waste water is reduced by using the dilute hydrofluoric acid solution discharged in the fluorine fixing reaction step as the steam for the dry vapor-phase reaction step.